In most automated color image reproduction systems such as a color photographic system or electronic color image sensing and display systems, the color balance of the reproduced image is automatically corrected, based upon measurements performed on the original color image (e.g. a color negative in tne case of color photography, or a recorded digital color image in the case of electronic color image systems). In general, the aim of the color correction is to cause the overall average color of the reproduced image to be a shade near gray (The Evan's "Gray World" Hypothesis in color photographic printing). The particular shade near gray toward which the image is adjusted is called the "aim point" of the color correction process. Since the location of the aim point in color space is a strong function of the color of the light that was used to illuminate the scene in the image (the scene illuminant), knowledge of the color of the scene illuminant enables a more accurate color correction to be made, thereby improving the appearance of the reproduced image, and for photographic printing, reducing the number of remakes required.
One approach to identifying the color of the scene illuminant involves a detector in the camera that samples the light coming from the light source and records a code on the film representative of the type of illuminant detected. The code may then be used in the image reproduction step (e.g. color printing) to adjust the aim point appropriately. This scheme has the drawback of adding complications to the camera, and is not generally employed in commercially available cameras. Additionally, the technique used to estimate the illuminant often relies upon some quality of the light other than a direct measurement of the wavelength, such as the flicker frequency of the light to distinguish for example between tungsten and fluorescent illumination.
Another approach, which is used in some commercial photographic printers, is to attempt to estimate the type of illuminant from the color photographic negative itself, either by the personal observation of an operator, which has the disadvantage of not being automatic; or by automatic classification equipment using LATD (Large Area Transmission Density) signals.
U.S. Pat. No. 4,339,517 issued July 13, 1982 to Akimoto provides an example of such a classification method. The classification method disclosed by Akimoto employs measurements of the color negative such as color LATD measurements, measurements of the hue at the maximum density point, the ratio of areas of particular colors, and the average hue of points having flesh color in an attempt to distinguish between under and over exposed negatives, negatives subject to color failure, negatives exposed in fluorescent or tungsten light, high color temperature or low color temperature negatives, and aged negatives. Each negative is color corrected according to the group to which it is assigned based on the classification method. The classification scheme proposed by Akimoto provides only a rough estimate of the color of the illuminant, and the estimate is subject to distortions by the contents of a scene (called color subject failure). To date, no method has been provided for accurately determining the color of the illuminant from the image itself, particularly in the presence of color subject failure. A method for more accurately detecting the color of scene illuminant would enable more accurate color correction, thereby improving the quality of photographic prints or electronic displays of images, and reducing the number of remakes in the case of photographic prints. It is the object of the present invention to provide a more accurate method of determining the color of the scene illuminant from a color image.